.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_futex_wait 3 "Sep 29, 2023" "liburing-2.5" "liburing Manual"
.SH NAME
io_uring_prep_futex_wait \- prepare a futex wait request
.SH SYNOPSIS
.nf
.B #include <linux/futex.h>
.B #include <unistd.h>
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_futex_wait(struct io_uring_sqe *" sqe ","
.BI "                              uint32_t *" futex ","
.BI "                              uint64_t " val ","
.BI "                              uint64_t " mask ","
.BI "                              uint32_t " futex_flags ","
.BI "                              unsigned int " flags ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_futex_wait (3)
function prepares a futex wait request. The submission queue entry
.I sqe
is setup for waiting on a futex at address
.I futex
and which still has the value
.I val
and with
.BR futex2 (2)
flags of
.I futex_flags
and io_uring futex flags of
.I flags .

.I mask
can be set to a specific bitset mask, which will be matched by the waking
side to decide who to wake up. To always get woken, an application may use
.B FUTEX_BITSET_MATCH_ANY .

.I futex_flags
follows the
.BR futex2 (2)
flags, not the
.BR futex (2)
v1 interface flags.

.I flags
are currently unused and hence
.B 0
must be passed.

This function prepares an async
.BR futex (2)
wait request. See that man page for details. Note that the io_uring futex
wait request is similar to the
.B FUTEX_WAIT_BITSET
operation, as
.B FUTEX_WAIT
is a strict subset of that.

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation. See the related man page for
details on possible values. Note that where synchronous system calls will return
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field.
.SH NOTES
Unlike the sync futex syscalls that wait on a futex, io_uring does not support
passing in a timeout for the request. Instead, applications are encouraged
to use a linked timeout to abort the futex request at a given time, if desired.
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR io_uring_prep_futex_waitv (3),
.BR io_uring_prep_futex_wake (3),
.BR io_uring_prep_link_timeout (3),
.BR futex (2)
.BR futex2 (2)
